“I’m only human.”
You have heard that excuse, right? Perhaps you’ve even made it!
In 1 Corinthians 3, it isn’t an excuse; it’s a problem.
John’s Lennon’s song “Imagine” struck a cord with most people because he spoke of a world we’d all like to live in: a world where all people live life in peace, where there is no greed or hunger, where people share. The reason this is a “dream” is because the behavior is so uncommon. It’s not the way humans live.
That’s what Paul means when he writes about people acting like “mere humans” and “worldly” and it’s why he criticizes the Christians in Corinth. With all their jealousy and quarreling, they are just “being human,” and that’s not acceptable. God didn’t create human beings to act these ways and he especially doesn’t intend that those in His family act that way either.
“I’m only human” is not an excuse for poor behavior, but even if it were, it would never be an appropriate excuse for Christians.